Typically, in an oil and gas well, surface safety valves (SSV) and subsurface safety valves (SCSSV) are used in the piping configuration. It is desirable when putting the well in service or for gaining access to the well to primarily open the SCSSV before opening the SSV. Similarly, if the well is in production and emergency situations arise, it is preferable to first close the SSV and then close the SCSSV. Prior systems have provided complex hydraulic circuits to regulate the sequencing of the opening and closing of the SSV and SCSSV. Prior art systems were complex to design, had numerous components, and took a significant amount of space to assemble and install. The multiplicity of components further added to the expense of assembly of these control systems and increased the possibility that maintenance would have to be performed.
It is, thus, an object of the invention to simplify prior hydraulic circuits by combining and eliminating certain components in order to achieve in a unitary valve the ability properly sequence the SSV and SCSSV in a prescribed manner. It is also an object of this invention to provide for orderly operation of the SSV and SCSSV in the event of failure of primary hydraulic/pneumatic pressure supply. It is also an object of this invention to provide a sequence valve which allows for quick emergency shutdown of the SSV in the event of an emergency.